Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Mott Insulator”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

The first-order structural transition in NiO at high pressure

Insulator-metal transition in NiO and concomitant collapse of spin and magnetism in the material are important subjects in studies of strong electronic correlations and as such they generate multiple theoretical and experimental efforts aimed at understanding the complicated physics of strongly correlated electronic systems. We report here the iso-structural transition in NiO accompanying the previously observed by us transition to the metallic state. These findings resolve some experimental controversies regarding the transition. We also discuss the latest theoretical results in detail and show that our data confirm some recent theoretical models. Given the simplicity of the material and its fundamental importance to the understanding of strongly correlated electronic systems, our findings should stimulate further experimental and theoretical efforts to understand this prototype Mott insulator.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Bandwidth Control and Symmetry Breaking in a Mott‐Hubbard Correlated Metal

Abstract In Mott materials strong electron correlation yields a spectrum of complex electronic structures. Recent synthesis advancements open realistic opportunities for harnessing Mott physics to design transformative devices. However, a major bottleneck in realizing such devices remains the lack of control over the electron correlation strength. This stems from the complexity of the electronic structure, which often veils the basic mechanisms underlying the correlation strength. This study presents control of the correlation strength by tuning the degree of orbital overlap using picometer‐scale lattice engineering. This study illustrates how bandwidth control and concurrent symmetry breaking can govern the electronic structure of a correlated SrVO 3 model system. This study shows how tensile and compressive biaxial strain oppositely affect the SrVO 3 in‐plane and out‐of‐plane orbital occupancy, resulting in the partial alleviation of the orbital degeneracy. The spectral weight redistribution under strain is derived and explained, which illustrates how high tensile strain drives the system toward a Mott insulating state. Implementation of such concepts can push correlated electron phenomena closer toward new solid‐state devices and circuits. These findings therefore pave the way for understanding and controlling electron correlation in a broad range of functional materials, driving this powerful resource for novel electronics closer toward practical realization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Electronic structure and small-hole polarons in YTiO 3

As a prototypical Mott insulator with ferromagnetic ordering, YTiO 3 (YTO) is of great interest in the study of strong electron correlation effects and orbital ordering. Here we report the first molecular beam epitaxy (MBE) growth of YTO films, combined with theoretical and experimental characterizations of the electronic structure and charge transport properties. The obstacles of YTO MBE growth are discussed and potential routes to overcome them are proposed. DC transport and Seebeck measurements on thin films and bulk single crystals identify p-type Arrhenius transport behavior with an activation energy of ~0.17 eV in thin films, consistent with the energy barrier for small hole polaron migration from hybrid density functional theory calculations. Hard x-ray photoelectron spectroscopy measurements show the lower Hubbard band at 1.1 eV below the Fermi level, whereas a Mott-Hubbard band gap of ~1.5 eV is determined from photoluminescence measurements. These findings provide critical insight into the electronic band structure of YTO and related materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Non-Centrosymmetric Sr 2 IrO 4 Obtained Under High Pressure

Sr 2 IrO 4 with strong spin-orbit coupling (SOC) and Hubbard repulsion (U) hosts Mott insulating states. The similar crystal structure, magnetic and electronic properties, particularly the d-wave gap observed in Sr 2 IrO 4 enhanced the analogies to cuprate high-$T_c$ superconductor, La 2 CuO 4 . The incomplete analogy was due to the lack of broken inversion symmetry phases observed in Sr 2 IrO 4 . Here, under high pressure and high temperature conditions, we report a non-centrosymmetric Sr 2 IrO 4 . The crystal structure and its noncentrosymmetric character were determined by single crystal X-ray diffraction and high-resolution scanning transmission electron microscopy (HR-STEM). The magnetic characterization confirms the Ir 4+ with $\textit{S}$ = 1/2 at low temperature in Sr 2 IrO 4 with magnetic ordering occurred at around 86 K, where a larger moment is observed than the ambient pressure Sr 2 IrO 4 . Moreover, the resistivity measurement shows three-dimensional Mott variable-range hopping existed in the system. Further, this non-centrosymmetric Sr 2 IrO 4 phase appears to be a unique material to offer further understanding of high-$T_c$ superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Auger spectroscopy beyond the ultra-short core-hole relaxation time approximation

Abstract We present a time-dependent computational approach to study Auger electron spectroscopy (AES) beyond the ultra-short core-hole relaxation time approximation and, as a test case, we apply it to the paradigmatic example of a one-dimensional Mott insulator represented by a half-filled Hubbard chain. The Auger spectrum is usually calculated by assuming that, after the creation of a core-hole, the system thermalizes almost instantaneously. This leads to a relatively simple analytical expression that uses the ground-state with a core-hole as a reference state and ignores all the transient dynamics related to the screening of the core-hole. In this picture, the response of the system can be associated to the pair spectral function. On the other hand, in our numerical calculations, the core hole is created by a light pulse, allowing one to study the transient dynamics of the system in terms of the pulse duration and in the non-perturbative regime. Time-dependent density matrix renormalization group calculations reveal that the relaxation process involves the creation of a polarization cloud of doublon excitations that have an effect similar to photo-doping. As a consequence, there is a leak of spectral weight to higher energies into what otherwise would be the Mott gap. For longer pulses, these excited states, mostly comprised of doublons, can dominate the spectrum. By changing the duration of the light-pulse, the entire screening process can be resolved in time.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Quantum fisher information reveals UV-IR mixing in the strange metal

The density-density response in optimally doped Bi 2 Sr 2 CaCu 2 O 8+$x$ has recently been shown to exhibit conformal symmetry. Using, the experimentally inferred conformal dynamic susceptibility, we compute the resultant quantum Fisher information (QFI), a witness to multi-partite entanglement. For a Fermi liquid, we find that the QFI grows quadratically as the temperature increases, consistent then with the phase space available for scattering in the standard theory of metals. By contrast, the QFI in a strange metal increases as a power law at as the temperature decreases, but ultimately extrapolates to a constant at T=0. The constant is of the form, ω$^{2Δ}_g$, where Δ is the conformal dimension and ω g is the UV cutoff which is on the order of the pseudogap. As this constant depends on both UV and IR properties, it illustrates that multipartite entanglement in a strange metal exhibits UV-IR mixing, a benchmark feature of doped Mott insulators as exemplified by dynamical spectral weight transfer. We conclude with a discussion of the implication of our results for low-energy reductions of the Hubbard model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Structural Manipulation of Phase Transitions by Self‐Induced Strain in Geometrically Confined Thin Films

Strain engineering is a well-known method often used to tune material properties in thin films. The most studied sources of strain are lattice mismatch and differential thermal contraction between the substrate and film. However, in materials which undergo a structural phase transition (SPT), a third and often overlooked source of strain may play a very significant role. If the substrate confines the area of the film, the SPT may induce stress which changes the evolution of the transition. This is a 2D analog of the isochoric phase transition between water and ice, where the freezing point drops below 0 °C. To illustrate this, the prototypical Mott insulator V 2 O 3 which has an SPT coupled to a metal–insulator transition is used to show how self-induced strain can drastically alter structural and electronic properties. This effect provides an elegant approach for mapping the phase diagram of the SPT and the transitions coupled to it. Moreover, the magnitude of self-straining is tunable by modifying the substrate morphology. Furthermore, this effect may be important for numerous materials which exhibit an SPT and are subjected to geometrical constraints.

Kalcheim, Yoav↗
DOE OSTI2022

Dirac Fermion Cloning, Moiré Flat Bands, and Magic Lattice Constants in Epitaxial Monolayer Graphene

Tuning interactions between Dirac states in graphene has attracted enormous interest because it can modify the electronic spectrum of the 2D material, enhance electron correlations, and give rise to novel condensed-matter phases such as superconductors, Mott insulators, Wigner crystals, and quantum anomalous Hall insulators. Previous works predominantly focus on the flat band dispersion of coupled Dirac states from different twisted graphene layers. Here in this work, a new route to realizing flat band physics in monolayer graphene under a periodic modulation from substrates is proposed. Graphene/SiC heterostructure is taken as a prototypical example and it is demonstrated experimentally that the substrate modulation leads to Dirac fermion cloning and, consequently, the proximity of the two Dirac cones of monolayer graphene in momentum space. Theoretical modeling captures the cloning mechanism of the Dirac states and indicates that moiré flat bands can emerge at certain magic lattice constants of the substrate, specifically when the period of modulation becomes nearly commensurate with the ($\sqrt{3}$ x $\sqrt{3}$) R30° supercell of graphene. Our results show that epitaxial single monolayer graphene on suitable substrates is a promising platform for exploring exotic many-body quantum phases arising from interactions between Dirac electrons.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Nanoscale Antiferromagnetic Domain Imaging using Full-Field Resonant X-ray Magnetic Diffraction Microscopy

The physical properties of magnetic materials frequently depend not only on the microscopic spin and electronic structures, but also on the structures of mesoscopic length scales that emerge, for instance, from domain formations, or chemical and/or electronic phase separations. However, experimental access to such mesoscopic structures is currently limited, especially for antiferromagnets with net zero magnetization. Here, full-field microscopy and resonant magnetic X-ray diffraction are combined to visualize antiferromagnetic (AF) domains of the spin–orbit Mott insulator Sr 2 IrO 4 with area over ≈0.1 mm 2 and with spatial resolution as high as ≈150 nm. With the unprecedented wide field of views and high spatial resolution, an intertwining of two AF domains on a length comparable to the measured average AF domain wall width of 545 nm is revealed. This mesoscopic structure comprises a substantial portion of the sample surface, and thus can result in a macroscopic response unexpected from its microscopic magnetic structure. In particular, the symmetry analysis presented in this work shows that the inversion symmetry, which is preserved by the microscopic AF order, becomes ill-defined at the mesoscopic length scale. We report this result underscores the importance of this novel technique for a thorough understanding of the physical properties of antiferromagnets.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Ohmic contact formation at the Au/Sr 3 Ir 2 O 7 interface

Here, using X-ray photoelectron spectroscopy (XPS), we show that Au forms an Ohmic contact with the surface of the narrow-bandgap Mott insulator Sr 3 Ir 2 O 7 thin films. XPS reveals no significant binding energy shift with increasing Au overlayer thickness on epitaxial Sr 3 Ir 2 O 7 (001) thin films, consistent with the linear current-voltage (I-V) behavior and indicative of negligible band bending. The absence of Schottky barrier formation at the Au/Sr 3 Ir 2 O 7 interface corroborates the metallic surface character of the Sr 3 Ir 2 O 7 thin film, as observed in angle-resolved photoemission spectroscopy (ARPES) studies, suggesting that the interface behaves effectively as a metal-on-metal contact. Furthermore, XPS points to the presence of distinct surface/interface and bulk electronic states for Sr 3 Ir 2 O 7 . This study provides critical material information for designing iridate-based electronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Change of Electrical and Transport Properties of Nickel Oxide by Carrier Concentration and Temperature through First-Principle Calculations

Abstract Nickel is typically used as one of the main components in electrical contact devices or connectors. Nickel oxide (NiO) is usually formed on the surfaces of electrodes and can negatively impact system performance by introducing electrical contact resistance. The thermal, electrical, and transport properties of NiO, as a Mott insulator or a p-type semiconductor, can be altered by operating and environmental conditions such as temperature and stress/strain by contact. In this study, we investigate the fundamental material properties of NiO through the first-principle calculations. First, we obtain and compare the lattice parameter, magnetic moment, and electronic structure for NiO via the WIEN2K simulations with four different potentials (i.e., GGA, GGA + U, LSDA, and LSDA + U). Then, using the WIEN2K simulation results with LSDA + U potential that produces a highly accurate bandgap for NiO, we calculate the electrical conductivity and electrical part of the thermal conductivity of nickel and NiO as a function of temperature and carrier concentration through the BoltzTraP simulations. Systematic simulation results revealed that the electrical conductivity relative to the relaxation time for NiO increases with the carrier concentration, while it shows a slightly decreasing trend with temperature under a fixed carrier concentration. By contrast, the electrical part of the thermal conductivity shows an increasing trend considering carrier concentration and temperature.

Shah, Nayem Md. Reza↗
DOE OSTI2025

Thermal Weight Determination and Interstate Coupling in State-Averaged ADAPT-VQE

Characterizing electronic thermal states at low temperatures is an important but challenging task in quantum chemistry and condensed matter physics, making it a prime candidate for a useful application in quantum computing. One of the most successful methods for state preparation on quantum computers is the Adaptive, Problem-Tailored (ADAPT) Variational Quantum Eigensolver (VQE), which has recently been generalized to treat excited states within a state-averaged framework as well as Gibbs states. In this work, we introduce Helmholtz-Optimized Thermal (HOT) ADAPT-VQE, an ancilla-free strategy for preparing Gibbs states that directly minimizes the Helmholtz free energy by targeting the dominant eigenstates of the thermal ensemble. We demonstrate the usefulness of HOT-ADAPT-VQE by predicting the free energy of two model systems with strongly correlated ground states: (1) the Fe 2+ cation in a magnetic field and (2) a [Cu 2 O 7 ] 10– fragment of the Mott insulator La 2 CuO 4 . Our results demonstrate that HOT-ADAPT-VQE significantly improves upon Gibbs-state estimates from multistate variants of ADAPT-VQE, often with substantially shallower quantum circuits, making it a promising candidate for thermal-state calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Charge ordering and structural transition in the new organic conductor {delta}'-(BEDT-TTF){sub 2}CF{sub 3}CF{sub 2}SO{sub 3}.

We report structural, transport, and optical properties and electronic structure calculations of the delta'-(BEDT-TTF) 2CF3CF2SO3 (BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene) organic conductor that has been synthesized by electrocrystallization. Electronic structure calculations demonstrate the quasi-one-dimensional Fermi surfaces of the compound, while the optical spectra are characteristic for a dimer-Mott insulator. The single-crystal X-ray diffraction measurements reveal the structural phase transition at 200 K from the ambient-temperature monoclinic P2(1)/m phase to the low-temperature orthorhombic Pca2(1) phase, while the resistivity measurements clearly show the first order semiconductor-semiconductor transition at the same temperature. This transition is accompanied by charge-ordering as it is confirmed by splitting of charge-sensitive vibrational modes observed in the Raman and infrared spectra. The horizontal stripe charge-order pattern is suggested based on the crystal structure, band structure calculations, and optical spectra.

Olejniczak, Iwona↗
DOE OSTI2024

Giant Anisotropic Magnetoresistance in Few-Layer α-RuCl 3 Tunnel Junctions

The spin–orbit-assisted Mott insulator α-RuCl 3 is proximate to the coveted quantum spin liquid (QSL) predicted by the Kitaev model. In the search for the pure Kitaev QSL, reducing the dimensionality of this frustrated magnet by exfoliation has been proposed as a way to enhance magnetic fluctuations and Kitaev interactions. Here, in this work, we perform angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin α-RuCl 3 crystals with various layer numbers to probe their magnetic, electronic, and crystal structures. We observe a giant change in resistance, as large as ~2500%, when the magnetic field rotates either within or out of the α-RuCl 3 plane, a manifestation of the strongly anisotropic spin interactions in this material. In combination with scanning transmission electron microscopy, this tunneling anisotropic magnetoresistance (TAMR) reveals that few-layer α-RuCl 3 crystals remain in the high-temperature monoclinic phase at low temperatures. It also shows the presence of a zigzag antiferromagnetic order below the critical temperature T N ≃ 14 K, which is twice the one typically observed in bulk samples with rhombohedral stacking. Our work offers valuable insights into the relation between the stacking order and magnetic properties of this material, which helps lay the groundwork for creating and electrically probing exotic magnetic phases such as QSLs via van der Waals engineering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Ultra-flat and long-lived plasmons in a strongly correlated oxide

Plasmons in strongly correlated systems are attracting considerable attention due to their unconventional behavior caused by electronic correlation effects. Recently, flat plasmons with nearly dispersionless frequency-wave vector relations have drawn significant interest because of their intriguing physical origin and promising applications. However, these flat plasmons exist primarily in low-dimensional materials with limited wave vector magnitudes (q< ~0.7 Å –1 ). Here, we show that long-lived flat plasmons can propagate up to ~1.2 Å –1 in α-Ti 2 O 3 , a strongly correlated three-dimensional Mott-insulator, with an ultra-small energy fluctuation (<40 meV). The strong correlation effect renormalizes the electronic bands near Fermi level with a small bandwidth, which is responsible for the flat plasmons in α-Ti 2 O 3 . Moreover, these flat plasmons are not affected by Landau damping over a wide range of wave vectors (q < ~1.2 Å –1 ) due to symmetry constrains on the electron wavefunctions. Our work provides a strategy for exploring flat plasmons in strongly correlated systems, which in turn may give rise to novel plasmonic devices in which flat and long-lived plasmons are desirable.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Experimental realization of an extended Fermi-Hubbard model using a 2D lattice of dopant-based quantum dots

The Hubbard model is an essential tool for understanding many-body physics in condensed matter systems. Artificial lattices of dopants in silicon are a promising method for the analog quantum simulation of extended Fermi-Hubbard Hamiltonians in the strong interaction regime. However, complex atom-based device fabrication requirements have meant emulating a tunable two-dimensional Fermi-Hubbard Hamiltonian in silicon has not been achieved. Here, we fabricate 3 × 3 arrays of single/few-dopant quantum dots with finite disorder and demonstrate tuning of the electron ensemble using gates and probe the many-body states using quantum transport measurements. By controlling the lattice constants, we tune the hopping amplitude and long-range interactions and observe the finite-size analogue of a transition from metallic to Mott insulating behavior. We simulate thermally activated hopping and Hubbard band formation using increased temperatures. As atomically precise fabrication continues to improve, these results enable a new class of engineered artificial lattices to simulate interactive fermionic models.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

1/4 is the new 1/2 when topology is intertwined with Mottness

In non-interacting systems, bands from non-trivial topology emerge strictly at half-filling and exhibit either the quantum anomalous Hall or spin Hall effects. Here we show using determinantal quantum Monte Carlo and an exactly solvable strongly interacting model that these topological states now shift to quarter filling. A topological Mott insulator is the underlying cause. The peak in the spin susceptibility is consistent with a possible ferromagnetic state at T = 0. The onset of such magnetism would convert the quantum spin Hall to a quantum anomalous Hall effect. While such a symmetry-broken phase typically is accompanied by a gap, we find that the interaction strength must exceed a critical value for this to occur. Hence, we predict that topology can obtain in a gapless phase but only in the presence of interactions in dispersive bands. These results explain the recent quarter-filled quantum anomalous Hall effects seen in moiré systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Crystallization of polarons through charge and spin ordering transitions in 1T-TaS 2

The interaction of electrons with the lattice in metals can lead to reduction of their kinetic energy to the point where they may form heavy, dressed quasiparticles—polarons. Unfortunately, polaronic lattice distortions are difficult to distinguish from more conventional charge- and spin-ordering phenomena at low temperatures. Here we present a study of local symmetry breaking of the lattice structure on the picosecond timescale in the prototype layered dichalcogenide Mott insulator 1T-TaS 2 using X-ray pair-distribution function measurements. We clearly identify symmetry-breaking polaronic lattice distortions at temperatures well above the ordered phases, and record the evolution of broken symmetry states from 915 K to 15 K. The data imply that charge ordering is driven by polaron crystallization into a Wigner crystal-like state, rather than Fermi surface nesting or conventional electron-phonon coupling. At intermediate temperatures the local lattice distortions are found to be consistent with a quantum spin liquid state.

36 MATERIALS SCIENCE↗